1. Field of the Invention
The invention relates to polycarbonate resins and more particularly to blends of polycarbonate resins with fluoropolymers such as polytetrafluoroethylene resin.
2. Brief Description of the Prior Art
Prior hereto it was known that synthetic and/or inorganic polymers such as glass could be coated with polytetrafluoroethylene (hereinafter referred to at times as "PTFE") to improve their soil release properties and to enhance their durability; see for example U.S. Pat. No. 3,968,297. In the latter patent, the method of coating comprises first applying a base coat of a tetravalent titanium oxide polymer, a zirconium oxide polymer or a tin oxide polymer. The base coat is then top coated with an unsintered aqueous dispersion of polytetrafluoroethylene particles.
It has also been proposed heretofore to coat elastomeric materials such as butadiene-acrylonitrile copolymers with unsintered polytetrafluoroethylene particles by first softening the polymer surface with a solvent and adhering the polytetrafluoroethylene particles to the softened surface; see for example U.S. Pat. No. 3,200,006. In U.S. Pat. No. 3,511,682 a method is described wherein the elastomer is coated with a thin film of polytetrafluoroethylene and then the film is sintered with an open flame on the elastomer surface.
More recently, the prior art has recognized the use of PTFE as an additive to polycarbonate resins, resulting in the enhancement of certain physical properties associated with the polycarbonate and useful in thermoforming processes. Polycarbonate resins having a degree of porosity are not just coated with the additive PTFE, but physically incorporate the PTFE into their particle structure. Early processes for incorporating PTFE into the polycarbonate resins included admixture and processing of the polycarbonate and the PTFE (in a latex) in a ball mill, a rubber mill, an extruder or a Banbury mixer; see for example the description in U.S. Pat. Nos. 3,294,871 and 3,290,795. In U.S. Pat. No. 3,005,795, it is suggested that admixture of the PTFE and the polycarbonate resin take place in "highly-fluid solutions and melts" of the base resin.
Most recently, it has been found advantageous to combine the PTFE and a polycarbonate resin by coprecipitation of the two resins. This is said to result in the ultimate in dispersion of the PTFE into the polycarbonate resin; see the description in the U.S. Pat. No. 4,753,994.
We have now found that the procedure followed for the physical admixture of the fluoropolymer, particularly PTFE, with a polycarbonate resin is important to a homogeneous incorporation of the fluoropolymer. A dispersing addition of the fluoropolymer in latex form to the polycarbonate resin, results in a thermoforming blend of particular physical properties, which molds into articles having certain improved properties.